1. Field of the Invention
The present invention relates to a feed-forward amplifier, which contributes to low-distortion amplification of electric signals, and a controller of the same.
The present invention is applicable to low-distortion amplification of multi-carrier signals or spread spectrum modulated signals. Expressed in more general terms, the present invention contributes to improvement in the quality of transmission signals in the fields of radio communications and cable communications. The feed-forward amplifier relating to the present invention is usable in base stations and repeaters for cellular telephones, broadcasting stations and relay stations for terrestrial wave digital television broadcasting systems, and various other systems requiring low-distortion amplification.
2. Description of the Related Art
Cellular telephone systems, in particular such systems using terrestrial waves, have a common configuration in which a large number of base stations are geographically dispersed from each other while users carry mobile stations. Furthermore, there are instances where a repeater is provided between the base station and the mobile station. Radio frequency amplifiers are provided at the base stations and at the repeaters to power-amplify the transmitting radio frequency signals to the mobile stations, to which strict low-distortion characteristics are required due to the reasons given in the following.
First, the input versus output characteristics of the amplifier inevitably exhibit some non-linearity of varying degrees. Distortion components generated by this non-linearity are of many types including harmonics, cross-modulation distortion, and intermodulation distortion. It is necessary to reject or suppress these distortion components that cause degradation of the signal quality. Some distortion components, such as harmonics, which often appear at frequencies considerably separated from the frequency band occupied by the input signal to the amplifier can be rejected by a filter provided at a stage subsequent to the amplifier. However, the remaining distortion components, such as cross-modulation distortion and intermodulation distortion having a frequency that is identical or extremely close to the frequency of the input signal to the amplifier are difficult or impossible to reject by such a filter. In particular, when the amplifier amplifies a plurality of carriers having frequencies extremely close to each other, neither the cross-modulation nor intermodulation distortion component can be rejected with a filter.
Cellular telephone systems are implemented according to mutually different standards in various countries in the world. Among these standards, the PDC (Personal Digital Cellular) standard cellular telephone system implemented in Japan, the GSM (Global System for Mobile communication) standard cellular telephone system implemented in many countries, such as in Europe, the IS-54/IS-136 standard cellular telephone system implemented in the US, and the EDGE (Enhanced Data-rates for GSM Evolution) standard cellular telephone system and the GPRS (General Packet Radio System) standard cellular telephone system, which are both called 2.5 generation cellular telephone systems, use many carriers having frequencies extremely close to each other. The CDMA (Code Division Multiple Access) standard cellular telephone system, which is currently being popularized or developed in various countries, transmits spread spectrum modulated signals. This type of system has been implemented in the U.S., Japan, and South Korea under the designation of cdmaOne or IS-95, and there are plans for its implementation under the designation of W-CDMA (Wideband CDMA), IMT-2000 (International Mobile Telecommunication-2000) or cdma2000.
As was clearly summarized in the above, a common characteristic of current and future cellular telephone systems is that the signals to be transmitted between a base station or repeater and a mobile station, or between a base station and a repeater include a plurality of components having frequencies extremely close to each other. More precisely, a signal including a plurality of frequency components having frequencies extremely close to each other is input to a radio frequency power amplifier (multi-carrier amplifier) in base stations or repeaters for the PDC, GSM, IS-54/IS-136, EDGE, and GPRS standards, or spread spectrum modulated signal is input to a radio frequency power amplifier in base stations or repeaters for the CDMA standard. Thus, the frequency components in the input signal are susceptible to cross-modulation or intermodulation. Furthermore, since the input signal includes many carriers or is spread spectrum modulated, distortion components are liable to appear due to the non-linearity of the amplifier. Namely, the radio frequency-power amplifier of base stations and repeaters for cellular telephone systems requires a scheme to suppress the generation of distortion components represented by the intermodulation distortion components, or an improvement to lower or eliminate distortion.
The radio frequency power amplifier of base stations and relay stations for terrestrial wave digital television broadcasting systems also requires a similar scheme. For example, since the terrestrial wave digital television broadcasting system planned for implementation in Japan transmits signals in which many carriers are multiplexed in accordance with the OFDM (Orthogonal Frequency Division Multiplex) standard, its amplifiers require an improvement to reduce or eliminate distortion.
However, it is impossible to realize an amplifier having ideal low-distortion characteristics, and it is often difficult to realize even an amplifier having near-ideal low-distortion characteristics due to constraints in terms of cost and circuit size. One approach for solving this problem is to add to the amplifier a circuit to reject or suppress the distortion components that are generated in the amplifier. This approach has been implemented heretofore in a form where a circuit is provided to detect the distortion components included in the output signal of the amplifier, and in accordance with the result thereof, to perform automatic control so as to minimize the distortion components included in the output signal of the amplifier. This type of amplifier having an additional circuit is called a distortion compensation amplifier.
One known type of distortion compensation amplification system of the prior art is the feed-forward system. Generally, the feed-forward system is adopted with an object to enable the maximum suppression or rejection, among the residual distortion components in the amplifier output, of the distortion components that are difficult to reject at a filter stage or the like, to enable the maintenance of desirable distortion component rejection and suppression performance even in the event of temperature variations or deterioration due to aging, and once this is achieved, to maintain and improve the quality of the transmission signals by obtaining a low-distortion amplified output. The distortion compensation amplifier adopting the feed-forward system is called a feed-forward amplifier.
The feed-forward amplifier comprises a main amplifier for amplifying signals, a distortion detection loop, which is a feed-forward loop for detecting the distortion components generated at the main amplifier, a distortion rejection loop, which is a feed-forward loop for rejecting or suppressing these distortion components from the output signal, and a controller for automatically controlling the operation of the distortion detection loop and the distortion rejection loop. Hitherto, various improvements and modifications have been proposed for the feed-forward amplifier. Relevant references include patents issued in Japan such as Japanese Patent Publication No. Hei 7-77330, Patent Nos. 2711413, 2711414, 2799911, 2804195, 2948279, 2945451, and 2945447, or corresponding laid-open publications including the original disclosure by one of the applications i.e. Japanese Patent Laid-Open Publication Nos. Hei 1-198809, 4-233811, 4-233809, 4-286209, 5-243880, 4-83406, 4-83407, and 4-70203.
FIG. 30 shows a typical configuration of the feed-forward amplifier in the prior art based on the disclosures in these patents and publications. The signal that is input by the feed-forward amplifier shown in FIG. 30 via an input terminal 1 from a circuit (not shown) of a previous stage, such as a modulator, is amplified by a main amplifier 5 and supplied from an output terminal 2 to a subsequent circuit such as an antenna or a filter previous to the antenna. Of the two types of feed-forward loops associated with this signal path, the distortion detection loop comprises a distributor 3, a vector adjustment circuit 4, the main amplifier 5, a delay circuit 6, and a directional coupler 7, and the distortion rejection loop comprises a gain adjustment circuit 8, a phase adjustment circuit 9, a sub-amplifier 10, a directional coupler 11, and a delay circuit 12. A controller 17 is a circuit for controlling the adjustment operations (to be explained hereinafter) in the distortion detection loop and the distortion rejection loop. A distributor 13, an oscillator 14, a narrow-band filter 15, and a receiver 16 form a circuit to achieve control by the controller 17 or form part of the controller 17.
The distributor 3 distributes an input signal from the input terminal 1 to the main amplifier 5 and the delay circuit 6. The main amplifier 5 amplifies the input signal that was distributed from the distributor 3 and supplies a resulting output signal A to the directional coupler 7. On the other side, the delay circuit 6 delays the signal that was distributed from the distributor 3 and supplies it to the directional coupler 7. The directional coupler 7 supplies on the one hand the output signal A from the main amplifier 5 to the directional coupler 11 via the delay circuit 12, and branches on the other hand part of the output signal A and couples it with a signal from the delay circuit 6. The directional coupler 7 supplies to the sub-amplifier 10 the signal obtained from coupling the part of the output signal A with the signal from the delay circuit 6. The sub-amplifier 10 amplifies the signal from the directional coupler 7 and supplies it to the directional coupler 11. The directional coupler 11 couples the signal delayed by the delay circuit 12 with the signal amplified by the sub-amplifier 10, and outputs a resulting signal B via the output terminal 2 to a circuit of a subsequent stage.
The two signals distributed by the distributor 3 do not include distortion components generated in the main amplifier 5, whereas the output signal A from the main amplifier 5 includes distortion components generated in the main amplifier 5. Therefore, among the two signals to be coupled at the directional coupler 7, one includes distortion components whereas the other does not. Furthermore, each of these two signals includes the main signal component, namely, the component corresponding to the signal that is input to the main amplifier 5 via the input terminal 1. Therefore, if the main signal components, which are the common components in these two signals, have the same amplitude and opposite phase at the signal coupling point within the directional coupler 7, the signal that is supplied to the sub-amplifier 10 from the directional coupler 7 becomes a signal including predominantly distortion components.
Furthermore, the signal supplied to the directional coupler 11 via the delay circuit 12 includes the distortion components generated by the main amplifier 5, and the signal amplified by the sub-amplifier 10, in general terms, includes only the distortion components generated at the main amplifier 5. Therefore, the two signals to be coupled at the directional coupler 11 each include the distortion components generated at the main amplifier 5. If these distortion components have the same amplitude and opposite phase at the signal coupling point within the directional coupler 11, the distortion components do not appear in the signal that is output via the output terminal 2 from the directional coupler 11.
To realize the amplitude and phase relationship between two signals at the signal coupling points within the directional couplers 7 and 11, the vector adjustment circuit 4 has been provided in the distortion detection loop, and the gain adjustment circuit 8 and the phase adjustment circuit 9 have been provided in the distortion rejection loop. The vector adjustment circuit 4 adjusts the component values for amplitude and phase in at least one of the distributed outputs from the distributor 3, which in effect adjusts the amplitude and phase relationship. The gain adjustment circuit 8 and the phase adjustment circuit 9 respectively adjust the amplitude and phase of at least one of the two signals that are output from the directional coupler 7. In FIG. 30, the distributed output to the main amplifier 5 and to the sub-amplifier 10 undergo adjustment whereas the signals via the delay circuits 6 and 12 may also undergo adjustment. This point is similar also in an embodiment of the present invention to be described hereinafter.
The vector adjustment circuit 4, the gain adjustment circuit 8, and the phase adjustment circuit 9 may be realized by a quadrature modulator, by a variable gain amplifier or a variable attenuator, and by a variable phase shifter, respectively. The vector adjustment circuit 4 may be realized in a combination of circuits similar to the combination of the gain adjustment circuit 8 and the phase adjustment circuit 9, the gain adjustment circuit 8 and the phase adjustment circuit 9 may be replaced by a circuit similar to the vector adjustment circuit 4, and the order of the gain adjustment circuit 8 and the phase adjustment circuit 9 can be transposed. Similar modification is also applicable to an embodiment of the present invention to be described hereinafter. The delay circuits 6 and 12 are means for compensating for the signal delays generated in the signal paths through the main amplifier 5 and through the sub-amplifier 10, which are provided in parallel to the delay circuits 6 and 12 respectively, and can be realized with various delay lines or members having equivalent functions. This possible modification or design is applicable also in an embodiment of the present invention to be described hereinafter.
In the heretofore known feed-forward amplifier, the adjustments in the vector adjustment circuit 4, as well as the gain adjustment circuit 8 and the phase adjustment circuit 9, are respectively set and controlled to optimum values so that the signal that is supplied to the sub-amplifier 10 includes predominantly distortion components and so that the signal appearing at the output terminal 2 does not include distortion components. So that an optimum control state is always obtained to cope with variations in ambient temperature and aging performance of component parts, control is performed heretofore using a pilot signal. In the publications and patents given earlier, at least two types of pilot signals are used. Namely, a first pilot signal for optimizing the distortion detection loop and a second pilot signal for optimizing the distortion rejection loop.
The first pilot signal is injected into the main signal in a stage prior to the distributor 3 so as to appear in both signals to be coupled at the directional coupler 7. If the state of the distortion detection loop has been optimized, the first pilot signal is canceled in the same manner as the main signal component by the signal coupling operation in the directional coupler 7, and the signal that is supplied to the sub-amplifier 10 should be a signal of only distortion components.
By testing whether or not the first pilot signal remains in the signal that is supplied to the sub-amplifier 10, and appropriately setting and updating the adjustment at the vector adjustment circuit 4 so as to minimize the residual pilot signal, it is possible to place the control state of the distortion detection loop in an optimum state and to have the signal that is supplied to the sub-amplifier 10 include only distortion components. It should be noted that the circuit for generating, injecting, and detecting the first pilot signal, and supplying control signals to the vector adjustment circuit 4 has been omitted from FIG. 30. This circuit is realized by a circuit configuration similar to a circuit (to be described hereinafter) relating to injection and detection of the second pilot signal as well as the optimized control of the distortion rejection loop.
On the other hand, the second pilot signal is injected into the signal at an arbitrary point on the signal path from the signal branch point within the distributor 3 to the signal branch point within the directional coupler 7 via the main amplifier 5 so as to appear in both of the two types of signals to be coupled at the directional coupler 11. In the example shown in FIG. 30, the second pilot signal of frequency f generated at the oscillator 14 is injected into the signal at the main amplifier 5. For example, the injection point is between stages of a plurality of amplifiers cascaded to form the main amplifier 5. If the control state of the distortion rejection loop is optimum, the second pilot signal is canceled in the same manner as the distortion components by the signal coupling operation in the directional coupler 11, and the signal appearing at the output terminal 2 should not include the distortion components.
To optimize the distortion rejection loop, the circuit shown in FIG. 30 executes the following operation. First, the distributor 13 that is provided between the directional coupler 11 and the output terminal 2 branches part of the output signal. Furthermore, the narrow-band filter 15 extracts from the branched signal the component of a sufficiently narrow band to which the frequency of the second pilot signal belongs. The extracted component is considered to be the second pilot signal that remained in the output signal from the output terminal 2. The receiver 16 detects its level. The controller 17 considers the detected level, namely, the amount remaining of the second pilot signal, to be an index indicating the amount remaining of the distortion component, on the basis of which the control signals are generated. By applying the generated control signals to the gain adjustment circuit 8 and the phase adjustment circuit 9, the controller 17 appropriately sets and updates the adjustments for amplitude and phase.
As described in the publications and patents given earlier, the controller 17 generally has the configuration shown in FIG. 31. In the circuit shown in FIG. 31, the level of received voltage detected by the receiver 16 is converted to digital data by an A/D converter 171 and input to a CPU 172. The digital data items obtained as a result of a process in the CPU 172 to determine the control signal values are converted to analog control signals (control voltages) by a D/A converter 174 and supplied to the gain adjustment circuit 8 and the phase adjustment circuit 9.
As described in the publications and patents given earlier, the process in the CPU 172 to determine the control signal values is normally realized as a step-by-step search process by alternately varying the amplitude adjustment and phase adjustment in steps. In this process, the CPU 172 alternately executes a gain varying operation and a phase varying operation. The gain varying operation is an operation to investigate the direction of gain change for a lower level detection value in the receiver 16 by varying the amplitude (gain) adjustment in the gain adjustment circuit 8 in steps of a predetermined amount, and vary the gain in that direction, and the phase varying operation is an operation to investigate the direction of phase change for a lower level detection value in the receiver 16 by varying the phase adjustment (phase shift) in the phase adjustment circuit 9 in steps of a predetermined amount, and vary the phase in that direction. The alternate execution of operations is performed because by simultaneously varying the gain and phase shift, it cannot be determined whether the change in the level detection value is due to the change in gain or the change in phase shift. After finding a combination of gain and phase shift where the level detection value is the lowest by repeating this sort of process, this is held as long as a significant change in the level detection value does not appear. When a significant change appears in the level detection value, the same search process is again executed. As a result, the CPU 172 prevents the distortion rejection loop from deviating from an optimum state due to variations in ambient temperature, aging performance of the component parts, or the like. In the figure, a memory 173 is used by the CPU 172.
However, the prior art described above has the following problems.
First, the optimum control state of the distortion rejection loop was established and maintained heretofore by executing the step-by-step search process with the CPU 172 shown in FIG. 31 or a circuit employing a similar signal processor, in the prior art. For this reason, after control is initiated or once the operating conditions change, a few seconds to approximately 10 seconds were required to optimize the control state of the distortion rejection loop. During this period, since the distortion rejection loop is not in an optimum control state, the distortion components and second pilot signal remain in the output signal, and various difficulties are created including the generation of unnecessary radiation from an antenna of a subsequent stage.
Next, in the prior art, since the optimization control of the adjustment in the distortion rejection loop was executed in accordance with the amount (the detected level) of residual second pilot signal, the maximum effect of rejecting and suppressing the distortion components was obtained at the frequency of the second pilot signal (refer to FIG. 33). The reason the optimum control state can only be realized at one point is mainly due to the fact that the component parts forming the feed-forward amplifier do not have perfectly flat frequency characteristics. Furthermore, the reason the point at which the optimum control state is realized is at the frequency of the second pilot signal is due to the fact that automatic control is performed so that the amount of residual second pilot signal reaches a minimum. Thus, on one hand, it can be said that it is desirable to set the frequency of the second pilot signal to a frequency sufficiently close to the operating band (spectrum distributed band in the CDMA standard) of the feed-forward amplifier. On the other hand, in order to preferably detect the amount of residual second pilot signal at the receiver 16, it is necessary to set the frequency of the second pilot signal so that the residual second pilot signal can be preferably separated and extracted at the narrow-band filter 15. Namely, it is desirable to set the frequency of the second pilot signal to a frequency that is sufficiently separated from the operating band of the feed-forward amplifier.
To reach a compromise between these conflicting conditions, heretofore, the frequency f (refer to FIGS. 32 and 33) of the second pilot signal was set outside the operating band of the feed-forward amplifier, and the difference xcex94f with the operating band of the feed-forward amplifier was set to maximize the effect of rejecting and suppressing the distortion components in the operating band of the feed-forward amplifier, yet to allow the residual second pilot signal to be extracted at the narrow-band filter 15. Apparently, the necessity of such delicate settings to obtain a balance between conflicting requests becomes a difficulty when designing and using the feed-forward amplifier. Furthermore, since sufficient narrowing of difference xcex94f is not allowed once the residual pilot signal can be preferably extracted, the effect of rejecting and suppressing the distortion components in the operating band of the feed-forward amplifier cannot be increased sufficiently to an extent close to the theoretical maximum limit.
The present invention is intended to solve the aforementioned problems by obviating the need to perform step-by-step adjustment control processing using a processing member represented by a CPU, by establishing an optimum control state in the distortion rejection loop in a time shorter than in the prior art, by realizing a more stable control of the distortion rejection loop, and by improving the distortion component rejection and suppression effect in the operating band of the feed-forward amplifier without delicate settings of the pilot signal frequency and with a simple circuit.
One aspect of the present invention is a controller used in the feed-forward amplifier comprising the main amplifier, the distortion detection loop for coupling part of the input signal to the main amplifier and part of the output signal from the main amplifier by adjusting a mutual relationship of amplitude and phase thereof so as to generate a distortion signal, and the distortion rejection loop for coupling the distortion signal and the output signal from the main amplifier by adjusting a mutual relationship of amplitude and phase thereof so as to generate a low-distortion output signal. The controller according to the present invention controls the above-mentioned adjustment operation by supplying control signals to the distortion detection loop and the distortion rejection loop so as to suppress distortion components generated by the main amplifier and remaining in the low-distortion output signal. Furthermore, another aspect of the present invention is the feed-forward amplifier that comprises the main amplifier, the distortion detection loop, the distortion rejection loop, and a controller according to the present invention.
The controller according to the present invention comprises an injection-side mixer, a detection-side mixer, and a synchronizing detector.
The injection-side mixer up-converts the base pilot signal using the local oscillation signal, to generate an upper-side pilot signal having a frequency equal to the sum of the frequency of the base pilot signal and the frequency of the local oscillation signal, and a lower-side pilot signal having a frequency equal to the difference of the frequency of the base pilot signal and the frequency of the local oscillation signal. The injection-side mixer injects the generated upper-side and lower-side pilot signals into the distortion detection loop so as to appear as respective signals to be coupled at the distortion rejection loop.
The state of the distortion rejection loop is considered to be in a non-optimum state when the upper-side and lower-side pilot signals remain in the low-distortion output signal. The detection-side mixer inputs part of the low-distortion output signal, and down-converts this low-distortion output signal to generate error signals of gain and phase which are in phase and at quadrature phase (xcfx80/2 [rad]) respectively, by using the same local oscillation signal that the injection-side mixer used during up-conversion and by using quadrature coupling.
The synchronizing detector performs synchronizing detection on the gain error signal and the phase error signal obtained in this manner with the base pilot signal as a reference signal. As a result, since the signals representing the error of the current control state with respect to the optimum control state are obtained as detected outputs of the synchronizing detector, the synchronizing detector supplies them as control signals to the distortion rejection loop (such as the gain and phase adjustment circuits or vector adjustment circuit). As a result, the state of the distortion rejection loop approaches an optimum state, and the distortion components remaining in the low-distortion output signal are rejected or suppressed.
The first point to be noted regarding the present invention is that the control signals are generated by performing synchronizing detection on the (second) pilot signal remaining in the low-distortion output signal with the base pilot signal as a reference signal. Thus, it becomes unnecessary to execute the step-by-step search process that was used in the prior art, and for this reason a CPU or processor becomes unnecessary. Furthermore, it is possible to simultaneously and automatically adjust both the gain and phase. Namely, according to the present invention, the time from the initiation of control until the establishment of the optimum control state or the time from a change in operating condition, such as temperature, until the re-establishment of the optimum control state can be shortened compared to the prior art in which the step-by-step search process is executed, thereby solving the problem of unnecessary radiation of the pilot signal and distortion components. Regarding this point, the present invention should be regarded as an application, modification, or improvement of the inventions already filed in Japan (Japanese Patent Application No. Hei 10-300667 and Japanese Patent Application No. Hei 11-191901) by the assignee of the present application.
Furthermore, in the prior art using only one type of pilot signal for distortion rejection loop optimization, the distortion component rejection and suppression effect reaches a maximum at the frequency of the pilot signal, thereby resulting in a problem where the maximum distortion component rejection and suppression effect cannot be obtained within the operating band of the main amplifier. The second point to be noted regarding the present invention is that two types of pilot signals (upper-side and lower-side pilot signals) are injected for distortion rejection loop optimization. The S distortion component rejection and suppression effect in the present invention reaches a maximum at-a frequency lower than the frequency of the upper-side pilot signal and higher than the frequency of the lower-side pilot signal, such as in the vicinity of the frequency of the local oscillation signal.
The third point to be noted regarding the present invention is that the dominant frequencies of the local oscillation signal and the base pilot signal are set so that at least part of the frequency band that is used by the feed-forward amplifier is included between the frequency of the upper-side pilot signal and the frequency of the lower-side pilot signal. According to the present invention, within the frequency band that is used by the feed-forward amplifier, the obtained distortion component rejection and suppression effect is higher at least in the part existing between the frequency of the upper-side pilot signal and the frequency of the lower-side pilot signal than at the frequencies of the upper-side and lower-side pilot signals.
In a preferred embodiment of the present invention, furthermore, the point where the distortion component rejection and suppression effect reaches a maximum is placed within the frequency band that is used by the feed-forward amplifier by setting the dominant frequencies of the local oscillation signal and the base pilot signal.
In a certain embodiment, the dominant frequency fL of the local oscillation signal is placed within the frequency band that is used by the feed-forward amplifier, and the dominant frequency fP of the base pilot signal has a frequency lower than the dominant frequency fL of the local oscillation signal. Thus, the dominant frequency of the upper-side pilot signal becomes fL+fP, and the dominant frequency of the lower-side pilot signal becomes fLxe2x88x92fP. As a result, the distortion component rejection and suppression effect reaches a maximum near frequency fL, which is within the frequency band that is used by the feed-forward amplifier. In particular, the dominant frequency fL of the local oscillation signal is preferably set to the center of the frequency band of the input signal to the main amplifier, and the dominant frequency fP of the base pilot signal is preferably set at a frequency equal to xc2xd or more of the frequency bandwidth that is used by the feed-forward amplifier so that a maximum or nearly maximum distortion component rejection and suppression effect can be obtained over the entire frequency band that the input signal to the main amplifier occupies.
In another embodiment, it is assumed the frequency band that is used by the feed-forward amplifier is divided into a plurality of channels, each having a predetermined channel width. In this embodiment, the dominant frequency fL of the local oscillation signal is placed within the guard band that has been provided between channels for channel separation, and the dominant frequency fP of the base pilot signal is a frequency that is a natural multiple of the channel width. As a result, the distortion component rejection and suppression effect reaches a maximum within a plurality of channels sandwiched by frequency fL+fP and frequency fLxe2x88x92fP, particularly in the channels in the vicinity of frequency fL.
The terminology xe2x80x9cdominant frequencyxe2x80x9d is used in the description above because an embodiment may spread the spectrums of the upper-side and lower-side pilot signals. Furthermore, although one preferred embodiment of the present invention has a first oscillator for generating the local oscillation signal and a second oscillator for generating the base pilot signal, they are not essential in embodying the present invention. For example, if oscillators oscillating at the same frequencies are provided in the circuit of an earlier stage such as a modulator, the required signals may then be as input from the oscillators.
The fourth point to be noted regarding the present invention is that the pilot signal for use in the optimized control of the distortion rejection loop, namely, the signal corresponding to the second pilot signal in the prior art, is generated from the up-conversion of the base pilot signal by mixing with the local oscillation signal. Therefore, by down-converting part of the low-distortion output signal into two signals, that are in phase and at quadrature phase, by the detection-side mixer using the same local oscillation signal, the upper-side and lower-side pilot signals remaining in the low-distortion output signal can be extracted and down-converted to the frequency of the base pilot signal, enabling the detection of control errors of gain and phase in the frequency band of the base pilot signal. The control signals are generated on the basis of detected control errors through the synchronizing detection of the down-converted signals. Since the same local oscillation signal can be used for both up-conversion and down-conversion and the base pilot signal can be used as a reference signal during synchronizing detection, and thus the fluctuation in upper- and lower-side pilot signal frequencies due to the fluctuation in the frequency of the base pilot signal or the local oscillation signal is cancelled by the down-conversion, the control signals can be generated without causing errors in control signals due to the frequency fluctuations in the frequency of the local oscillation signal or the base pilot signal. In other words, a low-cost design is possible with the use of oscillators that are not particularly stable.
Furthermore, members necessary for the injection of pilot signals and the detection of the remaining amount, and for the generation of control signals, are the injection-side mixer, the detection-side mixer, and the synchronizing detector. Thus, the circuitry relating to the present invention has a simple configuration compared to the circuitry of the prior art. According to the present invention, simplification of the circuit configuration is achieved, including a reduction in the number of mixers and detectors, and the circuit configuration of the controller relating to the present invention is simple when compared to the art according Japanese Patent Laid-Open Publication Nos. Hei 7-106861 and 8-56126, even when taking into consideration the filter, distributor, and directional coupler to be described hereinafter.
Japanese Patent Laid-Open Publication Nos. Hei 7-106861 and 8-56126 show the use of two types of second pilot signals having different frequencies from each other. However, since the two types of second pilot signals were generated respectively by separate oscillators, the number of oscillators, mixers, and detectors in the circuitry described in these publications is inevitably greater than the number in the present invention. Furthermore, the operation of injecting both the sum and difference frequency components appearing in the mixer output as pilot signals is not disclosed or suggested in these publications.
Furthermore, the levels of the upper-side and lower-side pilot signals to be injected can be set arbitrarily. In a preferred embodiment of the present invention, the levels are kept as low as possible in order to minimize circuit configuration, lower costs, and reduce power consumption. If the upper-side and lower-side pilot signals are at low levels, their residual levels in the low-distortion output signal are also low. Furthermore, since the control of the distortion rejection loop is performed with the aim of minimizing the remaining amount of the pilot signals as much as possible, their remaining amount (level) in the low-distortion output signal becomes lower as the optimum control state is approached. On the other hand, the main signal component is also included in the low-distortion output signal. The main signal component acts as a noise component or unnecessary component during detection of the residual pilot signals.
In a preferred embodiment of the present invention, the following process reduces the main signal component acting as the noise component or unnecessary component. Namely, a signal including the component corresponding to and having an opposite phase to the signal to be amplified by the main amplifier (but not including the upper-side and lower-side pilot signals) is added to the branched low-distortion output signal to be input to the detection-side mixer so as to cancel the main signal component from among the components included in the low-distortion output signal to be input to the detection-side mixer. Thus, the main signal component included in the input to the detection-side mixer, namely, the noise component or unnecessary component, is rejected or suppressed. As a result, the component corresponding to the main signal component, which acts as the noise or unnecessary component, is rejected or suppressed from the output after down conversion. Namely, according to a preferred embodiment of the present invention, the residual pilot signals can be detected in a stable manner in the form of error signals regardless of the fact that the residual pilot signals may be at extremely low levels. Furthermore, since the dynamic range of the detection-side mixer may be narrow, this also enhances the effects of minimizing circuit configuration, lowering costs, and reducing power consumption.
Furthermore, the upper-side and lower-side pilot signals in the present invention may be placed within or outside the frequency band that is used by the feed-forward amplifier. Although a single frequency signal may be respectively used for the upper-side and lower-side pilot signals, spread spectrum modulated signals may also be used. To prevent the upper-side and lower-side pilot signals from affecting the signal to be amplified and output, it is desirable to have the frequencies or spread spectrum bands of the upper-side and lower-side pilot signals outside the frequency band that is used by the feed-forward amplifier, or to use the spread spectrum modulated signals as the upper-side and lower-side pilot signals so that the pilot signals act as noise with respect to the component corresponding to the signal to be amplified at the feed-forward amplifier. To spread the spectrums of the upper-side and lower-side pilot signals to frequency bands having significant widths, a modulator may be provided to spread the spectrum of at least either the local oscillation signal or the base pilot signal.
Furthermore, a frequency where the distortion rejection and suppression effect reaches a maximum is definitely placed within the frequency band that is used by the feed-forward amplifier, by providing a modulator for spread spectrum modulation, or by setting the dominant frequencies of the local oscillation signal and the base pilot signal so that the frequencies of part or all of the spectrum distribution of the upper-side and lower-side pilot signal enter the frequency band of the signal to be amplified at the feed-forward amplifier. Thus, the distortion component rejection and suppression effect increases in the frequency band of the signal to be amplified at the feed-forward amplifier. In the embodiment employing this sort of frequency allocation, it is desirable to provide a circuit (already described) for performing anti-phase addition of part of the signal to be amplified at the feed-forward amplifier to the low-distortion output signal that is input by the detection-side mixer so as to stabilize the detection of the remaining amount of pilot signals.
It is desirable to provide a filter on the path supplying the error signals from the detection-side mixer to the synchronizing detector. This filter separates the frequency component at or including the base pilot-signal from the error signals obtained at the detection-side mixer, and supplies them to the synchronizing detector as error signals. Since the pilot signal in this embodiment is separated by the filter after down-conversion at the detection-side mixer, the selectivity of the filter for separating and extracting the residual pilot signal can be easily improved compared to the prior art in which the amount of the second pilot signal having a frequency adjacent to the operating band is directly detected. Thus, since the remaining amount of the pilot signal can be detected with more precision in the form of error signals, the precision of distortion compensation also improves.
Furthermore, when embodying the present invention using the upper-side and lower-side pilot signals that are spread in spectrum, in one embodiment, the reference signal that is input by the synchronizing detector becomes the signal that is spread in spectrum. When a filter for residual pilot signal separation and extraction is provided in this type of embodiment, the pass bandwidth of the filter is set wide according to the width of the spread spectrum band. In contrast, in an embodiment in which a modulator is arranged so as not to spread the spectrum of the reference signal and a circuit that performs despread spectrum modulation of the error signal is provided so that the synchronizing detector inputs an unmodulated signal, namely, a signal that is not spread spectrum modulated, the pass bandwidth of the filter for residual pilot signal separation and extraction can be set narrow to improve the selectivity.
Furthermore, the present invention can be understood to be an invention performing optimized control of the distortion rejection loop on the basis of a signal that has been rejected of the component corresponding to the signal to be amplified at the feed-forward amplifier among the components included in the low-distortion output signal. Namely, a second aspect of the present invention is a controller used in a feed-forward amplifier comprising a main amplifier, a distortion detection loop for coupling part of the input signal to the main amplifier and part of the output signal from the main amplifier by adjusting a mutual relationship of amplitude and phase thereof so as to generate a distortion signal, and a distortion rejection loop for coupling the distortion signal and the output signal of the main amplifier by adjusting a mutual relationship of amplitude and phase thereof so as to generate a low-distortion output signal, and controls the above-mentioned adjustment operation by supplying control signals to the distortion detection loop and the distortion rejection loop so as to suppress the distortion component generated at the main amplifier and remaining in the low-distortion output signal, and comprises means for branching part of the low-distortion output signal, means for performing anti-phase addition of part of the input signal to the main amplifier to part of the low-distortion output signal during or after branching, and means for generating the above-mentioned control signal for the distortion rejection loop on the basis of the signal from which has been rejected the component corresponding to the signal to be amplified at the feed-forward amplifier.